Implantable medical device with ventricular pacing protocol

ABSTRACT

An implantable medical device operates to promote intrinsic ventricular depolarization according to a pacing protocol. The medical device determines a course of action based upon the presence or absence of sensed ventricular activity. The device further determines whether that activity is properly conducted or the result of a PVC or nodal rhythm.

FIELD OF THE INVENTION

This invention relates implantable medical devices and more particularlyto implantable medical device for cardiac pacing.

BACKGROUND OF THE INVENTION

While a variety of pacing modes are available, dual chamberpacing/sensing (DDD) is commonly utilized. With a DDD mode, atrial andventricular events are both sensed. If an expected intrinsic event isnot sensed within a predetermined time window, an appropriate atrial orventricular pacing stimulus is delivered. This mode provides a greatdeal of control over the patient's cardiac rhythm and the timing (e.g.,the atrial-ventricular or AV delay) may be modified based upon manydifferent factors. One of the many benefits provided by the DDD mode isthe ability to maintain AV synchrony. That is, for any given atrialevent there will be a corresponding ventricular event, either intrinsicor paced.

Another beneficial feature is rate responsive (RR) pacing. With rateresponsive pacing, a demand sensor is provided that seeks to approximateactivity levels or physiological need from the patient and increase ordecrease the pacing rate accordingly. For example, an accelerometer isused to sense the patient's motion. As the patient is more active, theaccelerometer senses increased movement. This is recognized by theimplantable medical device (IMD), which could be, for example, animplantable pulse generator (IPG) or implantable cardioverterdefibrillator (ICD) with pacing capabilities, sometimes referred to as aPCD or pacemaker-cardioverter-defibrillator. In any event, theaccelerometer's signal causes the IMD to pace at a higher rate. Theassumption is that increased patient activity requires higher cardiacoutput and increasing the patient's heart rate (i.e., pacing rate) willlead to greater cardiac output. The higher the activity levels sensed,the higher the paced rate, up to a predetermined maximum rate. There area variety of demand sensors the may be employed such as, a minuteventilation sensor, blood oxygen sensor, QT interval, chemical sensors,motion/movement sensors, or any other device that will approximate oneor more demand parameters of the patient.

Typically, rate responsiveness is a positive feature that allowspatients to engage in higher activity levels than would be possible withfixed rate pacing. The combination of DDD with rate response is alsogenerally positive in that as the pacing rate is increased, the DDD modewill adjust parameters to assure proper timing throughout the cardiaccycle.

Recently, there has been a recognition that conducted or intrinsicventricular depolarizations are vastly preferable to ventricular pacingin general and pacing in the right ventricular apex in particular. Thedifficulty in facilitating this preference is that in a great manypatients, the intrinsic AV delay is so long that traditional DDD timingwill almost always deliver a ventricular pacing pulse. In order tominimize or greatly reduce ventricular pacing, a protocol had beenprovided that, in one embodiment, utilizes an atrial based timing modethat allows a full cardiac cycle to elapse without ventricular activity;thus providing the greatest opportunity to safely allow intrinsicconduction whenever possible. These protocols are described in U.S. Ser.No. 10/755,454, filed Jan. 12, 2004, entitled “Preferred ADI/R: APermanent Pacing Mode to Eliminate Ventricular Pacing While MaintainingBackup Support”, which is a continuation of U.S. Ser. No. 10/246,816,filed Sep. 17, 2002, entitled “Preferred ADI/R: A Permanent Pacing Modeto Eliminate Ventricular Pacing While Maintaining Backup Support”, whichis a continuation-in-part of U.S. Ser. No. 09/746,571, filed Dec. 21,2000, entitled “Preferred ADI/R: A Permanent Pacing Mode to EliminateVentricular Pacing While Maintaining Backup Support”, recently grantedas U.S. Pat. No. 6,772,005, all of which are herein incorporated byreference in their entireties.

As used herein, an atrial based pacing mode is a mode that is programmedto pace in the atria, but only to sense in the ventricles. True singlechamber atrial pacing would imply that only a single lead is present andventricular activity may not be sensed from within the ventricle norwould ventricular pacing be deliverable. In the present context wediscuss an IMD operating in an atrial based mode (e.g., AAI, AAIR, ADI,ADIR), but at least having ventricular sensing capabilities. Though notrequired, such a device would generally include ventricular pacing.However, in order to deliver ventricular pacing the device wouldtypically mode switch to a different mode, such as DDD, DDDR, DDI, orDDIR.

SUMMARY OF THE INVENTION

In one embodiment, the present invention includes a method of operatingan implantable medical device (IMD) that provides cardiac pacing andsensing, the method comprising operating the IMD in an atrial basedpacing mode with a ventricular pacing protocol (VPP), sensing forintrinsic ventricular activity, and determining the nature of the sensedventricular activity. The method further includes operating normally inthe atrial based pacing mode if the sensed ventricular activity isdetermined to be a properly conducted ventricular beat, initiating afirst response under the VPP if the sensed ventricular activity isdetermined to be a nodal rhythm, and initiating a second response underthe VPP if the sensed ventricular activity is determined to be apremature ventricular contraction (PVC).

In one embodiment, the VPP operates in an atrial based pacing mode andchanges to a dual chamber pacing mode for one cardiac cycle immediatelyfollowing a given cardiac cycle devoid of sensed intrinsic ventricularactivity, with a return to the atrial based pacing mode immediatelysubsequent to the one cardiac cycle.

In another embodiment, the VPP further includes an aggressiveness levelindicating a maximum number of cardiac cycles devoid of sensed intrinsicventricular activity in a given interval tolerated by the VPP.

The method may further include conducting a conduction check prior tooperating in the atrial based pacing mode to determine in intrinsicconduction is present, sensing for intrinsic ventricular activity, andanalyzing any ventricular activity sensed. The method further includesfailing the conduction check and operating in a dual chamber pacing modeif no ventricular activity is sensed, operating in an atrial basedpacing mode on an ongoing basis if the sensed ventricular activity is aproperly conducted ventricular beat, and failing the conduction checkand operating in the dual chamber pacing mode if the sensed ventricularactivity is an improper event.

In one example, the improper event is a PVC and in another a nodalrhythm.

In one embodiment, the first response includes operating in a dualchamber mode and initiating a pacing therapy to terminate the nodalrhythm for a first duration. It also includes performing a conductioncheck subsequent to the pacing therapy and operating in the atrial basedpacing mode if the conduction check is successful.

The method may also include initiating the pacing therapy for a secondduration, wherein the second duration is longer than the first duration,if the conduction check fails. It may also include performing a secondconduction check subsequent to the pacing therapy of the second durationand

operating in the atrial based pacing mode if the second conduction checkis successful. In one example, the method includes discontinuing the VPPif the second conduction check fails.

Alternatively, the method includes determining if the PVC is a firstoccurrence within a predetermined window and considering the firstoccurrence of a PVC within the window as a properly conductedventricular event.

In one example, the second response includes determining if the PVC is afirst occurrence within a predetermined window and modifying anaggressiveness level of the VPP if the PVC is the first occurrencewithin the window. Alternatively, the second response includesdetermining if the PVC is a first occurrence within a predeterminedwindow and operating under the VPP as if no ventricular activity weresensed if the PVC is the first occurrence. Alternatively, the secondresponse further includes determining if the PVC was preceded by anearlier PVC in an immediately prior cardiac cycle and changing the VPPto a least aggressive setting if the PVC was preceded by an earlier PVCin the immediately prior cardiac cycle.

In one example, the method further includes determining an AV intervalas defined by an atrial event and terminated by the PVC and returning toa more aggressive VPP setting after one cardiac cycle in the leastaggressive VPP setting if the AV interval is less than a predeterminedduration.

In another example, the second response further includes determining aPVC occurrence rate for a predetermined time period and operating theIMD in a less aggressive VPP setting if the total PVC occurrence rateexceeds a threshold. The method may additionally include determining ifthe PVC occurrence rate has exceeded the threshold for multipleconsecutive predetermined time periods and disabling the VPP if the PVCoccurrence rate has exceeded the threshold for multiple consecutivepredetermined time periods. In one example, disabling the VPP will onlyoccur if the VPP is currently in a least aggressive setting.

In one embodiment, the present invention is an implantable medicaldevice (IMD) comprising means for cardiac sensing and pacing.Additionally, the IMD includes means for distinguishing regularventricular events from improper ventricular events and means forcontrolling the means for cardiac sensing and pacing according to aVentricular Pacing Protocol (VPP) that respond in a first manner to theregular ventricular event and in a second manner for the improperventricular event.

In one embodiment, the improper ventricular events include prematureventricular contractions (PVC) or nodal rhythms. In one embodiment, thesecond manner further includes a PVC response and a nodal rhythmresponse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable medical device system in accordancewith an embodiment of the invention implanted in a human body.

FIG. 2 illustrates one embodiment of an implantable pacemaker devicesystem in accordance with the present invention coupled to a humanheart.

FIG. 3 is a block diagram illustrating the various components of oneembodiment of an implantable pacemaker device configured to operate inaccordance with the present invention.

FIG. 4 is a block diagram illustrating the various components of anotherembodiment of an implantable pacemaker device configured to operate inaccordance with the present invention.

FIG. 5A is a block diagram illustrating a Ventricular Pacing Protocol.

FIG. 5B is a block diagram illustrating a Ventricular Pacing Protocol.

FIG. 5C is a block diagram illustrating conduction check parameters fora Ventricular Pacing Protocol.

FIG. 6 is a flowchart illustrating a process for managing theVentricular Pacing Protocol based on physiological rate.

FIG. 7 is a block diagram illustrating various change options for theVentricular Pacing Protocol.

FIGS. 8-9 are flowcharts illustrating processes for determiningefficiency within a VPP.

FIG. 10 is a flowchart illustrating a process for VPP response tointrinsic AV delay parameters.

FIGS. 11A-11C are flowcharts illustrating a process for VPP response toimproper ventricular events.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified schematic view of one embodiment of implantablemedical device (“IMD”) 10 of the present invention implanted within ahuman body 6. IMD 10 comprises hermetically sealed enclosure 14 andconnector module 12 for coupling IMD 10 to pacing and sensing leads 16and 18 within heart 8.

FIG. 2 shows atrial and ventricular pacing leads 16 and 18 extendingfrom connector module 12 to the right atrium 30 and right ventricle 32,respectively, of heart 8. Atrial electrodes 20 and 21 disposed at thedistal end of atrial pacing lead 16 are located in the right atrium 30.Ventricular electrodes 28 and 29 at the distal end of ventricular pacinglead 18 are located in the right ventricle 32.

FIG. 3 is a block diagram illustrating the constituent components of IMD10 in accordance with one embodiment of the present invention, where IMD10 is pacemaker having a microprocessor-based architecture. IMD 10 isshown as including activity sensor or accelerometer 11, which is apiezoceramic accelerometer bonded to a hybrid circuit located insideenclosure 14. Activity sensor 11 typically (although not necessarily)provides a sensor output that varies as a function of a measuredparameter relating to a patient's metabolic requirements. For the sakeof convenience, IMD 10 in FIG. 3 is shown with lead 18 only connectedthereto; similar circuitry and connections not explicitly shown in FIG.3 apply to lead 16.

IMD 10 in FIG. 3 is programmable by means of an external programmingunit (not shown in the figures). One such programmer is the commerciallyavailable Medtronic/Nitatron Model 9790 programmer, which ismicroprocessor-based and provides a series of encoded signals to IMD 10,typically through telemetry via radio-frequency (RF) encoded signals orinductive coupling

As shown in FIG. 3, lead 18 is coupled to node 50 in IMD 10 throughinput capacitor 52. Activity sensor or accelerometer 11 is attached to ahybrid circuit located inside hermetically sealed enclosure 14 of IMD10. The output signal provided by activity sensor 11 is coupled toinput/output circuit 54. Input/output circuit 54 contains analogcircuits for interfacing to heart 8, activity sensor 11, antenna 56 andcircuits for the application of stimulating pulses to heart 8.Software-implemented algorithms stored in microcomputer circuit 58control the pacing rate.

Microcomputer circuit 58 comprises on-board circuit 60 and off-boardcircuit 62. Circuit 58 may correspond to a microcomputer circuitdisclosed in U.S. Pat. No. 5,312,453 to Shelton et al., herebyincorporated by reference herein in its entirety. On-board circuit 60includes microprocessor 64, system clock circuit 66 and on-board RAM 68and ROM 70. Off-board circuit 62 preferably comprises a RAM/ROM unit.On-board circuit 60 and off-board circuit 62 are each coupled by datacommunication bus 72 to digital controller/timer circuit 74.Microcomputer circuit 58 may comprise a custom integrated circuit deviceaugmented by standard RAM/ROM components.

Electrical components shown in FIG. 3 are powered by an appropriateimplantable battery power source 76 in accordance with common practicein the art. For the sake of clarity, the coupling of battery power tothe various components of IMD 10 is not shown in the figures. Antenna 56is connected to input/output circuit 54 to permit uplink/downlinktelemetry through RF transmitter and receiver telemetry unit 78.

As further shown in FIG. 3, VREF and Bias circuit 82 generates stablevoltage reference and bias currents for analog circuits included ininput/output circuit 54. Analog-to-digital converter (ADC) andmultiplexer unit 84 digitizes analog signals and voltages to provide“real-time” telemetry intracardiac signals and battery end-of-life (EOL)replacement functions. Operating commands for controlling the timing ofIMD 10 are coupled by data bus 72 to digital controller/timer circuit74, where digital timers and counters establish an overall escapeinterval of the IMD 10 as well as various refractory, blanking and othertiming windows for controlling the operation of peripheral componentsdisposed within input/output circuit 54.

Digital controller/timer circuit 74 is coupled to sensing circuitry 91,including sense amplifier 88, peak sense and threshold measurement unit90 and comparator/threshold detector 92. The embodiment of FIG. 4conforms substantially to that shown in FIG. 3, but incorporates adigital signal processor (DSP) 101 in lieu of sensing circuitry 90,including sense amplifier 88, peak sense and threshold measurement unit91, and comparator/threshold detector 92. DSP 101 receives signals,which may be amplified and processed, from lead 18. DSP 101 digitizesthe signals for analysis. DSP 101 may be coupled to micro-computercircuit 58 via data communications bus 72, permitting the micro-computercircuit to modify the processing characteristics of the DSP. Also, DSP101 may provide signal data to micro-computer circuit 58 for addedanalysis or control functions. An example of an implantable medicaldevice incorporating a DSP for ECG signal analysis is disclosed in U.S.Pat. No. 6,029,087 to Wohlgemuth, the entire content of which isincorporated herein by reference.

Digital controller/timer circuit 74 is further preferably coupled toelectrogram (EGM) amplifier 94 for receiving amplified and processedsignals sensed by lead 18. In the embodiment of FIG. 3, sense amplifier88 amplifies sensed electrical cardiac signals and provides an amplifiedsignal to peak sense and threshold measurement circuitry 90, which inturn provides an indication of peak sensed voltages and measured senseamplifier threshold voltages on multiple conductor signal path 67 todigital controller/timer circuit 74. An amplified sense amplifier signalis then provided to comparator/threshold detector 92. Alternatively,similar signals can be generated by DSP 101 for transmission to digitalcontroller/timer circuit 74. The electrogram signal provided by EGMamplifier 94 is employed when IMD 10 is being interrogated by anexternal programmer to transmit a representation of a cardiac analogelectrogram.

Output pulse generator 96 provides pacing stimuli to patient's heart 8through coupling capacitor 98 in response to a pacing trigger signalprovided by digital controller/timer circuit 74 each time the escapeinterval times out, an externally transmitted pacing command is receivedor in response to other stored commands as is well known in the pacingart.

In some embodiments of the present invention, IMD 10 may operate invarious non-rate-responsive modes, including, but not limited to, AAI,ADI, upon whether the VPP is capable of mode switching to anatrial-based mode. Assuming a mode switch to the atrial-based mode (ADIRas used in this example), the IMD 10 will mode switch to and operate inADIR (186) for one complete cardiac cycle. If intrinsic ventriculardepolarization is not sensed, then the IMD 10 will mode switch (185)back to the dual chamber mode or in this embodiment, DDDR. If intrinsicventricular depolarization is sensed, then the IMD 10 operates (190) inthe atrial-based pacing mode. As described above, the varying levels ofaggressiveness determine what constitutes sufficient AV conduction toallow the IMD 10 to remain in the atrial based pacing mode. If,according the VPP parameters, there is insufficient intrinsic conduction(192), then the IMD 10 will mode switch to and operate in DDDR (182).This is distinct from the iterative returns to a dual chamber mode toprovide ventricular pacing after a cardiac cycle devoid of a ventriculardepolarization with a predetermined return to the atrial based mode in asubsequent cycle.

Whether in the DDDR mode or in ADIR mode, the IMD 10 will mode switch toDDIR (194) in the event that an atrial tachyarrhythmia (195) is sensed.When a normal sinus rhythm (196) is restored, the IMD 10 will modeswitch to the DDDR mode and proceed accordingly.

When the VPP is programmed to a mild level of aggressiveness, and use ofthe atrial based pacing mode is not permitted, then the IMD 10 willoperate in the DDDR mode. The IMD 10 utilizes the AV search function toidentify and promote intrinsic conduction, as described above.

FIG. 5C is a block diagram that illustrates various tests available inthe VPP to determine whether intrinsic AV conduction is present. Whenthe VPP will only permit an AV search (505), then the IMD 10 willperiodically extend the duration of the programmed AV interval. Whilethe numerical value may vary depending upon the parameters of the VPP,there will be some maximum permissible AV interval that the IMD 10 canutilize. Thus, one conduction test permits the IMD 10 to set the AVinterval to this maximum value (510). For any given cardiac cycle, useof this maximum value will provide the highest level of promotion forintrinsic conduction in the mild VPP.

When performing such a conduction check, intrinsic conduction might notimmediately return as the cardiac tissue and conduction pathway mighttake a period of time to normalize in a particular patient. Where thismay be DDD, DDI, VVI, VOO and WT modes. In other embodiments of thepresent invention, IMD 10 may operate in various rate-responsive modes,including, but not limited to, ADIR, AAIR, DDDR, DDIR, VVIR, VOOR andVVTR modes. Some embodiments of the present invention are capable ofoperating in both non-rate-responsive and rate responsive modes

IMD 10 may also be a pacemaker-cardioverter-defibrillator (“PCD”)corresponding to any of numerous commercially available implantablePCD's. Various embodiments of the present invention may be practiced inconjunction with PCD's such as those disclosed in U.S. Pat. No.5,545,186 to Olson et al., U.S. Pat. No. 5,354,316 to Keimel, U.S. Pat.No. 5,314,430 to Bardy, U.S. Pat. No. 5,131,388 to Pless and U.S. Pat.No. 4,821,723 to Baker et al., all hereby incorporated by referenceherein, each in its respective entirety.

FIG. 5A illustrates a Ventricular Pacing Protocol (VPP) (150) thatprovides programmable or selectable settings for the IMD 10 that reduceor minimize ventricular pacing to varying degrees. The IMD 10 isoperable in a traditional mode (e.g., DDD, DDDR, etc.). When VPP isenabled, the physician will select the VPP having the desired level(155) of aggressiveness. As will be described, the IMD 10 may move froma given VPP level to a less aggressive VPP level, may switch back to themore aggressive VPP level, or may switch out of VPP, but will generallynot engage a VPP level higher (in aggressiveness) than selected by theprogramming physician. For example, in one embodiment more aggressiveVPP levels are tolerated by a given patient during sleep; thus, the IMD10 adjusts to this level based on a sensor indication of sleep and/or atime selection (e.g., nocturnal hours). The VPP levels are described forillustrative purposes only and it should be appreciated that more orfewer levels could be provided consistent with the present invention.Furthermore, relative terms are used to describe the VPP levels forillustrative purposes only and are non-limiting. The examples providedherein will indicate for consistency that “higher” levels are “more”aggressive than “lower” levels of VPP. Aggressiveness relates to thetolerance that the VPP will have for missed or delayed ventricularevents.

The least aggressive VPP setting in this embodiment is the mild VPP(160). Mild VPP functions similarly to DDD or DDDR in that AV synchronyis maintained. This is, mild VPP (160) will normally deliver aventricular pacing pulse in every cardiac cycle unless certain intrinsicventricular activity is sensed. In order to do so, an AV delay isprovided. At the end of the AV delay, the ventricular pacing pulse isdelivered absent appropriate intrinsic ventricular activity. In order topromote intrinsic ventricular depolarization, the AV interval in themild VPP can be rather long, e.g., 350-500 ms, as compared to standardsettings in DDD or DDDR. Thus, for a patient with intact but prolongedintrinsic AV conduction, the longer AV interval will permit intrinsicventricular depolarization. If no intrinsic conduction occurs, the AVinterval is shortened to achieve hemodynamic optimization. Periodically,a search is conducted by lengthening the AV delay to determine if AVconduction has returned. The frequency of conducting such a search andthe length of the AV interval are variables that the IMD 10 or theprogramming physician can change, either manually or automatically bythe IMD 10 depending upon the rate of success. The search may berealized by extending the AV delay for a single beat or for more beatsand the extension may become active entirely at once or will graduallybe achieved over several beats. In addition, when pacing occurs at theend of the extended AV delay, either as a result of a failed scan orafter a period of intrinsic AV conduction when the AV conduction fails,this may occur for one or for more cardiac cycles before returning tothe shorter value for more optimized hemodynamics.

In the moderate VPP setting (170), the IMD 10 operates in an atrialbased pacing mode, such as AAI, ADI, AAIR, ADIR or the like. For anygiven cycle in the atrial based pacing mode, ventricular depolarizationwill only occur if it is intrinsic. That is, no ventricular pacing pulsewill be delivered. If a cycle occurs without a ventricular event, theIMD 10 then mode switches to DDD, DDI, DDDR, or DDIR for the nextcardiac cycle. For simplicity, these traditional modes will becollectively referred to as dual chamber pacing modes. It should beappreciated that a reference herein to any specific mode such as DDDR,is meant to be illustrative and any of the traditional dual chamberpacing modes may be interchanged as appropriate.

In the cycle subsequent to the one devoid of a ventricular event, theIMD 10 will operate in DDD and deliver a ventricular pace (unlessprecluded by intrinsic activity as is standard in the DDD modality). Forthe next subsequent cycle (or after a certain number of beats or periodof time in the dual chamber modality, in certain embodiments), the IMD10 mode switches back to the atrial based pacing mode. This activity isall part of the moderate VPP (170); that is, these mode switches arepart of the protocol not a departure from the protocol.

Another aspect of the VPP is that it monitors the number of missedventricular events. If more than a predetermined number of missedventricular events occur within a specified period or within a specifiednumber of beats, then the IMD 10 mode switches to DDD and remains in DDDfor a prolonged period of time. Periodically, the VPP will initiate aconduction check by mode switching to the atrial based pacing mode forone cycle to determine if intrinsic conduction has returned. If it has,the process operates as previously described. If not, the IMD 10 modeswitches to DDD and again remains there until the next conduction check.If intrinsic activity occurs and precludes a ventricular pace duringDDD, the VPP, in one embodiment, will cause the IMD 10 to switch to theatrial based pacing mode.

In the moderate VPP (170), the protocol will tolerate up to one missedventricular event in twenty consecutive cardiac cycles. In theaggressive VPP 180, the protocol will tolerate up to one missedventricular event in four consecutive cardiac cycles. When theseparameters are exceeded, the IMD 10 mode switches to DDD for a prolongedperiod of time, as previously explained. These numerical embodiments aremerely illustrative examples and are not limiting. That is, for a givennumber of cardiac cycles, the more cycles tolerated without ventricularactivity, the more aggressive the protocol is labeled. The particularnumber chosen may be selected for any number reasons and those providedabove are merely exemplary. Similarly, more “levels” of aggressivenessmay be defined by simply providing more selectable tolerance values.Finally, rather than designating levels, the VPP aggressiveness may beadded as a programming parameter with the particular number of cyclestolerated numerically selectable.

FIG. 5B is a block diagram that illustrates operation of the IMD 10 in aVPP. For illustrative purposes, the IMD 10 is described as initiallyoperating in the DDDR (182) mode. At some point in time, as defined bythe VPP, and as early as the first cardiac cycle operated under the VPPthe IMD 10 will perform a conduction check. The type of conduction checkperformed will depend upon the level of aggressiveness of the VPP and ingeneral depends of concern, the VPP provides intrinsic conductionpromotion parameters that may be selected. Thus, when conducting an AVsearch by extending the interval to the maximum duration (510), afailure to sense intrinsic conduction in the first cardiac cycle willnot terminate the test. That is, the VPP maintains the maximum intervalfor some predetermined number of cardiac cycles to provide anopportunity for intrinsic conduction to return. The number of cyclespermitted may be standardized, adjusted based upon patient specificdata, or programmed by a clinician. Of course, in this embodiment, ifintrinsic conduction fails to return for a given cycle then the IMD 10provides a ventricular pace at the termination of the extended AVinterval. If no intrinsic conduction is sensed after the permittednumber of cycles, the AV interval is returned to the shorter duration.

Another intrinsic conduction promotion parameter is to increment (515)the AV interval, rather than immediately extending the interval to itsmaximum value. For example, the “standard” DDD AV interval may be 150 msfor a given patient. For the first increment, the AV interval isextended to e.g., 200 ms, then 250 ms, etc. up until the maximum value,e.g., 500 ms. Each increment may be a single cardiac cycle at eachinterval, a static predetermined number of cycles at each AV interval,or a dynamic number of cycles at each interval. Whichever methodology isemployed, if intrinsic AV conduction is sensed, the AV interval ismaintained at that or a longer setting until a loss of intrinsicconduction occurs sufficient to trigger the VPP to provide the shorterAV interval.

The IMD 10 may utilize a VPP that permits operation in an atrial basedpacing mode (520). When a test is performed to check for intrinsicconduction, the VPP mode switches the IMD 10 to the atrial based pacingmode, e.g., ADIR, for one cycle; thus, the heart is given the completecardiac cycle to facilitate intrinsic conduction. If during any point ofthat cardiac cycle, intrinsic conduction occurs, then the IMD 10determines that it can operate in the atrial based pacing mode, asdescribed.

As indicted above, intrinsic conduction in the cardiac tissue orconduction pathway may, in some cases, take some time to return to itsfullest potential. Thus, another intrinsic conduction promotionparameter is to incrementally extend the AV interval before operatingfor a full cycle (530) in the atrial based pacing mode. For example, theIMD 10 increases the AV interval from its normal setting to e.g., 200ms; in a subsequent cycle 250 ms; in subsequent cycle 300 ms; eventuallyproviding an AV interval of 500 ms. Then, the IMD 10 mode switches tothe atrial paced pacing mode for one complete A-A interval. In thismanner, the underlying or intrinsic conduction is given a greater periodof time (over several cycles) to present itself absent rigorousventricular pacing with short AV interval. Of course, intrinsicconduction may be sensed during any of these cycles and if so, the IMD10 will mode switch to the atrial based pacing mode. That is, theconduction test need not move through the entire sequence and include atest cycle in the atrial base mode. Intrinsic conduction in any cycle isconsidered a positive result and lead to a mode switch to the atrialbased mode.

In another embodiment, incrementally stepping through AV intervals ofprogressively longer length still requires a testing cycle in theatrial-based mode. When intrinsic conduction is sensed, the testingprocess may complete subsequent AV interval extensions and then operatein the atrial-based mode for one A-A interval. Alternatively, thetesting process may bypass subsequent AV interval extensions and switchto the atrial based pacing mode for a test cycle upon sensing intrinsicconduction during the extended AV interval.

This distinction between requiring the test cycle in the atrial basedpacing mode or mode switching into the atrial based pacing mode basedupon sensed intrinsic conduction during an AV interval relates to theeffect that a subsequent cycle, lacking intrinsic conduction will haveon the IMD 10. That is, in an ADIR test cycle, a lack of conduction willresult in a mode switch to and continued operation in DDDR for someperiod of time. If however, the VPP is operating ADIR, multiple cardiaccycles will have to be devoid of intrinsic conduction to cause the VPPto mode switch to DDDR for an extended period of time.

For example, the aggressive VPP tolerates an absence of intrinsicconduction in one out of four cycles. Thus, in such an example, ifintrinsic conduction was anomalously detected during a test cycle, thefirst cycle of operation in ADIR will be devoid of intrinsic conduction.In the subsequent cycle, the IMD 10 operates in DDDR and a ventricularpace is delivered. In the next subsequent cycle, the IMD 10 operatesagain in ADIR and is devoid of intrinsic conduction. Now, two of threeand hence two of four consecutive cardiac cycles lack intrinsicconduction and the VPP will mode switch to DDDR for an extended periodof time.

During periods of exercise, particularly strenuous exercise, there is aconstant and high demand for cardiac output. Naturally, the atrial ratewill increase to provide that increased cardiac output. This may occurintrinsically; that is, the SA node will pace the heart at anappropriately elevated rate in response to various factors of, e.g., theautonomic nervous system. Alternatively, the IMD 10 may utilize anactivity sensor or other physiological proxy sensor and determine thatrate should be increased. In response, the atrial pacing rate iselevated appropriately and up to an upper rate limit.

In either case, the A-A interval becomes correspondingly shorter. Inresponse, the AV node will normally decrease the intrinsic AV delay;thus, ventricular depolarization will occur temporally closer to thepreceding atrial depolarization. With an IMD operating in DDDR, the AVand VA interval are appropriately controlled. In the VPPs discussed,intrinsic ventricular depolarization is preferred and in certain cardiaccycles ventricular pacing is essentially precluded.

For various reasons, the intrinsic AV delay for a given patient may notdecrease appropriately or may actually increase in response to anelevation in the atrial rate. Alternatively, or in addition thereto,various degrees of conduction block may occur during this period eitherstatistically by chance or as a result of the increase in activity,rate, or a related parameter. In these cases, while operating undercertain of the VPPs, a cardiac cycle may be devoid of ventriculardepolarization. This results in an elongated R-R interval covering themissed beat(s). As indicated, this is normally non-symptomatic duringperiods or rest and normal activity. In the more aggressive VPP levels,a higher number of missed ventricular events is permitted. This couldlead to a cycle of missed events that is tolerated by the protocol butmay be perceivable to the patient during strenuous exercise. Forexample, if every fifth cycle were devoid of ventricular activity, theaggressive VPP, in one embodiment, would continue.

FIG. 6 is a flowchart illustrating a process to address the effects thatan elevated heart rate may have with one of the ventricular pacingprotocols. While operating in one of the VPPs, the IMD 10 monitors (200)the cardiac rate. The cardiac rate as used herein includes either theintrinsic rate or the sensor driven or device determined rate based uponprogramming values or activity levels. Typically, this refers to theatrial rate, either paced or sensed; however, sensing the ventricularrate or other cardiac timing parameters could also be utilized todetermine the cardiac rate. While decisions could be based uponindividual measurements, i.e., rate as calculated by an A-A timinginterval, the determined (200) rate is generally based upon a runningaverage over a predetermined number of cardiac cycles. The predeterminednumber of cycles is selected large enough such that anomalous ratevariations do not over-affect the process and low enough that the IMD 10responds quickly enough to actual sustained increases in cardiac rate.While not limited as such, the predetermined number of cycles may be onthe order of 5, 10, 20, or more cycles.

Alternatively, the averaged rate may be based upon the physiologicalrate as defined and utilized in various Medtronic Vitatron products. Insummary, the physiologic rate is a tracking rate that varies on abeat-to-beat basis with, e.g., the atrial rate. As variance in thetracked rate occurs, the physiologic rate only varies by a set numericalvalue (e.g., 2 bpm) in the direction indicated. For example, if thetracked rate drops by 10 bpm (based on a given A-A or V-V interval), thephysiologic rate is dropped by 2 bpm. One the next cycle, thephysiologic rate can move another 2 bpm and so on. Thus, largebeat-to-beat variances are not immediately reflected in the physiologicrate and this provides a smoother value to utilize. There is a limit tothe amount of change that is not tracked. For example, if at any timethe physiologic rate varies from the tracked rate by greater than, e.g.,15 bpm then this is determined to be a classified event and processed assuch. A classified event would be, for example, an indication of theonset of tachycardia. As used herein, the cardiac rate would include avalue determined either based upon a beat to beat value, by averagingover a given number of cycles, by utilizing a physiologic ratealgorithm, or by some other smoothing function.

As the cardiac rate is determined (200), that rate is compared (210)with a threshold value. The threshold value is meant to delineate restor normal activity levels from elevated activity such as exercise andparticularly strenuous exercise. In one embodiment, the threshold valueis 95 bpm. Depending upon the patient or physician concerns, thethreshold value may be selected from any value between the upper andlower rate limits. That is, certain patients may be more or lesstolerant of these affects at varying rates and the threshold can be setaccordingly. Younger patients implanted with an IMD 10 may have upperrate limits on the order of 190 bpm; thus, their threshold might behigher than, for example an elderly patient. Rather inactive patientsmay be strenuously exerting themselves at 75 bpm. Thus, the thresholdwill be determined for a patient and may be varied as the patient'scondition changes. In general, the threshold will be between 75 bpm and120 bpm, and typically between 85 bpm and 100 bpm.

As an alternative to setting a fixed threshold value, the thresholdvalue could be programmed to be the lower rate limit (LRL) plus somefixed constant; the LRL plus a variable dependant upon certain factors(age, condition, activity, previous rate trends, etc.), the upper ratelimit (URL) minus some fixed constant, or the URL minus a variabledependant upon certain factors (age, condition, activity, previous ratetrends, etc.). Thus, the threshold value will change in response toother programming or mode variations such as changes to the upper orlower rate limits.

If the cardiac rate is at or below the threshold, then the IMD 10continues to monitor the rate (200) and will continue to operateaccording to the VPP. If however, the cardiac rate exceeds the threshold(210), then the ventricular pacing protocol is changed (220).

How the VPP is changed will depend upon which VPP is in place when thecardiac rate exceeded the threshold. In general, the issues presentedabove are of concern when more aggressive protocols are in place; thatis, an entire cycle may pass without ventricular activity. It is fromthese protocols that a change would be made.

One change (220) is to switch from the current VPP to anotherappropriate (non-VPP) pacing mode, such as DDDR. Alternatively, thechange (220) would be switching to a less aggressive VPP or switching toa VPP that would ensure 1:1 AV synchrony at a hemodynamically optimalsetting. That is, intrinsic ventricular depolarization is stillpreferred and promoted to the extent possible; however, ventricularpacing will be delivered to assure each cardiac cycle includesventricular depolarization and that there is a sufficient VA intervalbased upon the cardiac rate.

FIG. 7 illustrates examples of changes to the VPP that may be made(220). When changing the VPP at (220), the IMD 10 will move (222) fromthe current VPP employed to the least aggressive VPP available (e.g.,Mild 160, FIG. 5A). When more levels are provided that assure 1:1 AVsynchrony, then the change (210) could be to the least aggressive VPPthat assures this (224), but not necessarily the least aggressive(overall) VPP. As another change option, the VPP could be stepped downby one (or any predetermined number) level of aggressiveness (225).Thus, if the VPP were operating the in aggressive VPP (180), then theIMD 10 would change to the moderate VPP (170). Likewise, if the IMD 10were in the moderate VPP (170), the change would be to the mild VPP(160). In any of these embodiments, the changed VPP may be reevaluated(226) to determine if it is appropriate for the patient's cardiac rate.If additional changes are required, the VPP may again be changed asindicated herein. Another change (220) that could be made is to move outof the VPP into a non-VPP mode.

Referring again to FIG. 6, when the change (220) has been made, the IMD10 continues to monitor (230) the cardiac rate. A determination is made(240) as to whether the averaged rate has returned to or fallen belowthe threshold. Assuming the rate is still above the threshold, the IMD10 continues to operate in the mode or protocol that it was changed toat (220) and continues to monitor the averaged rate (230). When it isdetermined (240) that that averaged rate has returned to or fallen belowthe threshold, then the IMD 10 will return (250) to the ventricularpacing protocol in place prior to the elevation in cardiac rate.

Alternatively, though not separately shown, the threshold employed at(240) could be a different value than the threshold employed at (210)and/or the predetermined number of cycles utilized to determine theaverage rate could be set differently at (230) then at (200). That is,there may be a desire to assure a return to normal or resting activitylevels prior to resuming the ventricular pacing protocol rather thansimply a pause or temporary reduction during, e.g., a strenuous aerobicworkout. This would simply be a matter of physician programmingpreference, as the present invention will appropriately address thesituation if the cardiac rate varies about the threshold.

One mechanism to prevent repetitive transitions across the lowerthreshold is to require a return to a threshold (240) that issignificantly lower than the upward threshold (210). For example, thelower threshold may be 10, 15, 20 or more beats per minute lower thanthe upper threshold. Another mechanism would be to require that thecardiac rate remain at or below the lower threshold (240) for a longerperiod of time before considering the rate to have returned below thethreshold. That is, not simply a binary threshold crossing to make thedetermination. The duration may be based on a predetermined period oftime (e.g., 10-60 seconds, 1-5 minutes, 5-10 minutes, etc.). Inaddition, a determination that the lower threshold has been crossedcould require both the lowered threshold value and the durationrequirement. In any event, a determination will eventually be made thatthe cardiac rate has returned to a value below the lower threshold value(240).

This return (250) to the prior ventricular pacing protocol is presumedto be an appropriate course of action. The patient is presumed tobenefit from returning to the previously established level ofaggressiveness with the VPP. Of course, for any number of reasons, thepatient's condition may no longer tolerate that level of aggressivenesseven though the cardiac rate has returned below threshold. For example,partial or complete conduction block could occur that would then requireventricular pacing. Thus, the return to the more aggressive VPP ishandled like any other conduction check employed by these protocols. Forexample, the AV delay could be progressively elongated to search forintrinsic conduction, a switch may be made to ADIR for one completecycle, or similar parameters would be employed to assure that there isunderlying intrinsic conduction justifying the level of aggressivenesswith the VPP.

It should be appreciated that one benefit of the VPPs is the reductionof ventricular pacing and the promotion of intrinsic conduction. Duringthe course of strenuous exercise and the reduction of aggressiveness inthe VPP, the patient could receive ventricular pacing that may have beenexcluded in a more aggressive mode; however, this will generallypreclude missed ventricular events and longer R-R intervals.Furthermore, patients having the IMD 10 will typically only exercise forrelatively short periods of time; thus, ventricular pacing is onlyminimally increased. For those patients who may exercise more frequentlyor who otherwise do not encounter such symptoms at increased cardiacrates, the VPP could be programmed to remain in effect, even above thethreshold.

When employing one of the ventricular pacing protocols (VPP), adetermination of effectiveness can be made based upon certain objectiveparameters. Effectiveness relates to how many paced ventricular eventshave been avoided.

As indicated, the more aggressive VPPs will tolerate a missedventricular event for an entire cycle; pace in a subsequent cycle andreturn to an atrial based pacing mode for the next subsequent cycle. Ifthe patient has developed complete conduction block and this pattern iscontinued, the effect is to only have ventricular depolarization inevery other cycle. In other words, the ventricular rate is halved withrespect to the atrial rate. Thus, for a patient with complete conductionblock, ventricular pacing is greatly reduced, i.e., halved; however,this is generally not a hemodynamically sound pacing methodology, in andof itself. This halving effect illustrates that a measure of theeffectiveness of a VPP does not rest solely with the number ofventricular pacing pulses averted.

Similarly, the number of mode switches or number of switches betweenvarious VPPs does not necessarily determine effectiveness. Rather,effectiveness is determined by the successful switch into a VPP, whereinsuccess is defined by having a number of cardiac cycles with intrinsicAV conduction and intrinsic ventricular depolarization. As such,effectiveness is defined according to the present invention as:Effectiveness (E)=Nbeats/Nswitches

where Nbeats=the number of cycles following a switch with intrinsicventricular depolarizations and

where Nswitches=the number of switches from the atrial based pacing modeto a ventricular pacing mode (e.g., DDD or DDDR)

Thus, the larger the value of E, the greater the effectiveness. As such,effectiveness is not simply the “gain” in the number of intrinsicventricular depolarizations, nor is it simply the number of switchesmade. As an example, if over a given time period there were 100 switchesmade, and there were 10 beats with intrinsic conduction for each switch,then the patient has received 1000 less ventricular pacing pulses thanmight have occurred absent the VPP. If, over the same period of time,there were 10 switches made with 100 beats with intrinsic activity foreach, the same 1000 ventricular paces have been avoided. However,according to the above formula the effectiveness is much greater in thelater case.E=10/100=0.1  Case 1:E=100/10=10  Case 2:

In other words, the VPP is 100 times more effective in the second casethan in the first. The same formula for effectiveness can be applied toAV search attempts rather than actual mode switches. That is Nswitchesis replaced with Nsearches.

The effectiveness value is used to determine the frequency at which tocheck for intrinsic conduction. That is, when the effectiveness valuehas been high, the IMD 10 will more aggressively and more frequentlycheck for intrinsic conduction because it has been shown to be of valueto this patient. Conversely, when the effectiveness value is low for agiven patient, less frequent conduction checks are made because there isless expectation of success.

Furthermore, the effectiveness value E is utilized by the IMD 10 tooptimize the VPP settings. That is, based upon other sensor data and/orother patient data such as cardiac output, ejection fraction, strokevolume, heart rate, patient symptoms, cardiac timing parameters or thelike, the IMD 10 can determine when the various VPPs are mostappropriate and their most appropriate settings for a givenphysiological situation. For example, if effectiveness E routinely dropsduring periods of exertion that include elevated heart rates that stillfall below the threshold (210), then the IMD may lower the threshold.Similarly, the effectiveness may be very high during sleep and lowerduring other periods. Thus, the IMD 10 may be more aggressive inemploying the VPP during the night and less aggressive during the dayfor a given patient.

The effectiveness value is also useful to a physician programming theIMD 10 as the value forms a basis by which therapies and/or settings maybe suggested by an analysis of the information or by automatedevaluation of collected data. Often, this will be patient dependant. Forexample, in an otherwise healthy patient an efficiency value of 0.1(e.g., saving 10 ventricular paced events per switch, on average) wouldbe beneficial, but might not be critical. Conversely, in a heart failurepatient such a reduction in ventricular pacing may be extremelybeneficial, thus suggesting more aggressive use of the VPP.

Similarly, the collection of efficiency values over time will illustratetrends for a given patient. That is, the available VPPs may maintain alevel of effectiveness over time and thus, their selection will remainappropriate. Alternatively, a patient's condition may change ordeteriorate and the VPP might be come less effective, as evidenced by adownward trend in the efficiency value. If this occurs, the frequency ofconduction checks could be reduced accordingly. Conversely, ifefficiency is trending upward, then that frequency might be increased tofurther exploit the benefits of the VPP.

FIG. 8 is a flowchart that illustrates the process for determiningefficiency (E) for a VPP over a given time period. At some point intime, the IMD 10 switches (300) into the atrial based pacing mode. Dueto this event, a switch counter is incremented (310). Over the course ofthe cardiac cycle, the IMD 10 monitors (340) for intrinsic ventricularactivity. If ventricular activity is sensed, the cycle in consideredsuccessful and a (successful) beat counter is incremented (350). The IMD10 returns to monitoring for intrinsic ventricular events (340) insubsequent cycles. If during any such cycle, there is no sensedventricular activity, then the IMD 10 mode switches to, e.g., DDDR (330)and delivers a ventricular pace if necessary. Whether on the next cycleor at some subsequent time, the IMD 10 will attempt to return to theatrial based pacing mode (300). This will likewise increment (310)switch counter by one and the process is repeated.

When an efficiency value is desired (360), the output value from thebeat counter is divided by the output value of the switch counter. Thecalculated value is the efficiency E for the relevant time period. Therelevant time period is the total number of cycles that have occurredsince resetting the values to zero for the switch counter and the beatcounter. Alternatively, the relevant time period is a given period oftime based on the internal clock circuit of the IMD 10. In this manner,the values for the switch and beat counters can be correlated to a fixedtime and efficiency values (E) can be calculated for relevant timeperiods of varying lengths.

FIG. 9 illustrates a process similar to that of FIG. 8, for calculatingthe efficiency of AV search attempts. At some point in time, the IMD 10extends (370) the AV delay to determine in intrinsic conduction ispresent. Due to this event, an search counter is incremented (375). Overthe course of the AV delay, the IMD 10 monitors (380) for intrinsicventricular activity. If ventricular activity is sensed, the cycle inconsidered successful and a (successful) beat counter is incremented(390). The IMD 10 returns to monitoring for intrinsic ventricular events(380) in subsequent cycles. If during any such cycle, there is no sensedventricular activity during the AV delay, then the IMD 10 delivers aventricular pace at the end of the AV delay. The IMD 10 will then take asubsequent step in the AV search process, such as returning to thenormal AV delay (385). At some future time, the IMD 10 will againconduct an AV search (370). This will likewise increment (375) searchcounter by one and the process is repeated. Though not separately shown,the subsequent step may be to further extend the AV delay at (385),rather then returning to the normal, shorter AV delay. In this manner,progressively long AV delays may be attempted up to a maximum delay.

When an efficiency value is desired (395), the output value from thebeat counter is divided by the output value of the search counter. Thecalculated value is the efficiency E for the relevant time period.

Another consideration in optimizing VPP and pacing in general is theeffect of an elevated atrial rate on cardiac output. In an atrial basedpacing mode, where ventricular pacing is generally not provided, anelevated pacing rate might not lead to higher cardiac output. Cardiacoutput is the measure of blood pumped for a given period of time. Thecomponents of cardiac output include heart rate and stroke volume. Thesetwo components are not necessarily independent of one another and strokevolume is not as dynamically variable as heart rate. An increase in rateoften reduces the stroke volume because of reduced filling times. Withrespect to heart rate, Wenckebach block may also occur, thereby reducingthe effective ventricular rate despite an increase in the atrial rate.In addition, the heart requires more energy at the higher rate, thusincreasing its own demand for oxygenated blood. Therefore, in order foran elevated pacing rate to beneficially increase cardiac output, theincrease gained from the rate component must offset the reduction instroke volume and higher physiological demands that result.

In response to an elevated heart rate (paced or intrinsic), the AV nodeshould normally adjust the AV delay to ensure proper timing, i.e. the AVdelay becomes shorter as heart rate increases. In some patients, thisdoes not occur or the delay actually increases and in either case, theAV delay may become excessively long with respect to the rate. Thismight result from a deficiency in the autonomic response mechanism, theAV node, or along the conduction pathway. Furthermore, since anartificial sensor perceives the demand for rate response or programmingfunctionality increase rate artificially (e.g., preventive pacingparameters such as atrial overdrive pacing), the autonomic response,even if functional, might not affect the AV node with sufficient speed.

As the AV delay increases (relative to the A-A interval), the VA delaycorrespondingly decreases. When the VA becomes too short (VAencroachment) for a prolonged period of time, negative effects mayresult. The ventricles may be fully contracted when the subsequentatrial contraction occurs. Alternatively, the ventricles may be onlypartially relaxed during that subsequent atrial contraction. The resultis that stroke volume is moderately to severely reduced, Wenckebachblock occurs, and/or atrial pressures may increase. When this occurs,the elevated atrial rate does not actually increase cardiac output andmay in fact reduce it This issue is of concern in two circumstances. Thefirst is where there is a physiological need for an elevated pacing rate(e.g., activity sensor indicates exercise), but a problem exists thatprevents proper intrinsic AV delay timing. The other circumstance iswhere the pacing rate is non-physiologic; that is, a preventive pacingregime such as atrial overdrive pacing or when the rate response ismisinterpreting sensory input to increase demand. In these cases, the AVnode might not reduce the AV delay or may actually lengthen theintrinsic AV delay. Thus, the benefits sought by a non-physiologicallydetermined rate (e.g., overdrive pacing) might still be obtained;however, the other issues presented may also result.

FIG. 10 is a flowchart illustrating VPP response to sensed AV delayparameters. The IMD 10 monitors (400) the atrial rate, whether sensed orpaced. While the following process has applicability in othersituations, this embodiment is described with respect to the IMD 10operating in a VPP and utilizing an atrial based pacing mode. The IMD 10determines if the atrial rate is elevated (405). Such an elevation mayoccur due to physiologic need, perceived physiologic need (e.g.,activity sensor), non-physiological pacing regimes (e.g., overdrivepacing), or various arrhythmias or other conditions. While notexplicitly limited, if the elevation occurs due to an arrhythmia such asatrial flutter or atrial fibrillation, other mechanisms are typicallyused to respond.

The IMD 10 monitors (410) the duration of the intrinsic AV delay. Thatis, the interval from the paced or sensed atrial event to the sensedventricular depolarization. Of course, such monitoring may occurcontinuously, but for purposes of the present process, is referred towhen the atrial rate is elevated (405).

The IMD 10 will determine (415) if the monitored AV delay is too longfor the current rate. In one embodiment, the AV delay is considered toolong if its duration exceeds the duration of the AV delay at a loweratrial rate. That is, if the AV delay increased when the atrial rateincreased, then the longer AV delay is deemed problematic. In anotherembodiment, the AV delay is consider too long if it is equal to orlonger than the AV delay at a lower atrial rate. In another embodiment,the AV interval is deemed too long if the corresponding VA intervalfalls below a predetermined threshold. Such a threshold may be aprogrammed value on the order of 100, 125, 150, or 200 ms or longer. Inanother embodiment, the acceptable AV delay is determined based upon alook-up table or formula correlating atrial rate with acceptable AVdelay programmed into the IMD 10.

If the IMD 10 determines the AV delay is acceptable for a given atrialrate, then the device will continue to monitor (410) the intrinsic AVdelay until the atrial rate has returned to the lower value. If the AVdelay is deemed too long, then the IMD 10 will take action (420) inresponse. One or more of the following options may be taken as theresponsive action (420), either based upon user programmable selectionsor device determination.

For any given cardiac cycle, the subsequently scheduled atrial pace maybe delayed (425) by a fixed value or by a fixed value (430) along with asafety margin. Effectively, this will elongate the VA interval in thepresent cycle. The length of the delay for the atrial pace may be afixed value anytime it is employed or a value corresponding to theelevated atrial rate (405). That is, if the atrial pace is delayed, thefixed value may be, for example, 30 ms. Alternatively, different delaytimes are available depending upon the atrial rate. Rather than using afixed value, the IMD 10 may make a dynamic determination as to when topermit the scheduled atrial pace. That is, the intrinsic ventricularevent is sensed and then the atrial pace is permitted to occur. Bydelaying the subsequent atrial pace, many of the issues presented aboveare addressed. That is, a sufficient interval is provided between theventricular depolarization and the subsequent paced atrial event. Such amodification will have the effect of altering the atrial rate, but isnot an adjustment of the atrial rate per se. In other words, theeffective atrial rate may vary on a beat-to-beat basis; an action istaken during a cardiac cycle to affect that cycle; and/or intrinsicventricular timing affects subsequent atrial pacing.

Another action that can be taken is to reduce (435) the actualatrial-pacing rate. This may be done in lieu of or in addition todelaying an atrial pace for a current cycle. For example, the MD 10 maydelay one or more atrial paces and then if required, lower the overallatrial rate. Alternatively, the atrial rate is lowered without delayinga scheduled atrial pace for the current cycle.

In lowering the atrial rate, the A-A interval is extended in durationand is uniform over time. Thus, a given AV interval will correspond to asmaller percentage of the A-A interval and similarly provide for alonger VA interval. Typically, an adjustment to the atrial rate willtake effect in a cycle subsequent to that when the change isimplemented, whereas delaying an atrial pace would affect the currentcycle and could result in varying A-A intervals over a short span if theoverall programmed atrial rate remains the same. In other words, adelayed atrial pace is a variance from the programmed atrial rate thatwill affect the achieved pacing rate for a small number of cardiaccycles whereas changing the programmed pacing rate is a global change.

The atrial rate may be reduced (435) to the resting rate (445) or tosome intermediate value (450) between the resting rate and the currentelevated atrial rate. The value chosen will depend upon the pacingmethodology responsible for the elevated rate. For example, with atrialoverdrive pacing, a determination has been made that an elevated pacingrate will be therapeutic in response to some sensed parameters. Thus,the decision is whether to now forgo overdrive pacing and return to aresting rate (or sensor rate) or conduct or attempt to conduct overdrivepacing at a rate lower than the current elevated atrial rate.

In making such determinations, the relevant data is stored (440) inmemory and may be telemetered to an external device for physicianreview. This data can provide guidance as to what atrial rates lead toprolonged AV intervals for a given patient. In response, this data canbe used to alter or decrease the slope (as one possible parameter) ofthe rate response for a device. That is, as the activity sensorindicates a perceived need for cardiac output, the resultant elevationin the atrial pacing rate is proportionally lower after the modificationto rate response.

The predetermined rate of a pacing regime can be lowered (460). Forexample, for atrial overdrive pacing, one regime may cause the atrialrate to increase 20 bpm over the sensed rate. After an adjustment basedupon the data (440), overdrive pacing will occur at a rate 10 bpm overthe sensed rate, for example. In addition, the new value can beiteratively determined based upon multiple attempts that result instored data (440). In the overdrive example, the overdrive pacing ratemay be adjusted from a 20 bpm increase to a 15 bpm increase. The IMD 10may determine when implementing overdrive pacing at 15 bpm, that the AVinterval is longer than desired. Thus, overdrive pacing is againmodified to a 10 bpm increase. In other words, the adjustments need notbe predetermined or static but may be dynamically determined based onpatient data.

As another response to the stored data (440) a pacing parameter otherthan rate may be modified. For example, the threshold values fordetermining when to enable overdrive pacing can be adjusted to reducethe occurrence of that therapy. Similarly, such a regime may be disabled(470). That is, overdrive pacing, rate response or similar therapies orprocesses may be selective disengaged to prevent their use will the VPPis in effect.

Another action that the IMD 10 may take (420) is to terminate, on ashort or long term basis, the VPP. For example, the IMD 10 may modeswitch (475) to DDD or DDDR. If desired, the VPP can be modified so asto include the parameters discussed; that is, the determination of theAV delay when the atrial rate is elevated. When the AV delay is longerthan permitted, then the VPP will treat the determination in the samemanner that a loss of conduction is treated and switch from the atrialbased pacing mode to a dual chamber pacing mode until such time as aconduction check is warranted. The timing with respect to when toconduct a subsequent conduction check may be identical to that used forloss of conduction or may be modified so perform the conduction checkonly when the atrial rate has lowered to an appropriate level.

In the various embodiments described above, the VPPs seek to promoteintrinsic AV conduction, intrinsic ventricular depolarization, andreduce ventricular pacing. In order to do so, the IMD 10 sensesventricular events. In a broad sense, any intrinsic ventricular eventsensed between two atrial events meets the above-described criteria andallows the IMD 10 to switch to or remain in the most aggressivelypermissible configuration of the VPP. For example, when operating inADIR, any ventricular activity sensed at any time during the A-Ainterval can be labeled as a ventricular event that indicates AVconduction and ventricular depolarization. In many cases, this broadcategorization functions efficiently and effectively.

It should be appreciated that not all sensed ventricular activityrepresents properly conducted, synchronous ventricular depolarization.For example, a premature ventricular contraction (PVC) or a nodal rhythmmay occur. When sensed by the IMD 10, such improper events meet thebroad definition of a ventricular event occurring between two atrialevents. However, these improper ventricular events contribute lesshemodynamically than properly conducted beats. Thus, the presentinvention provides a mechanism to address improper ventricular events inthe context of a VPP.

Various mechanisms are available to determine whether a ventricularevent is a properly conducted event or an improper event such as a PVCor nodal rhythm. The timing of the event within an interval (certainwindows of time are more likely to indicate a PVC), the lack of apreceding sensed P wave (more likely to indicate an ectopicdepolarization), a minimal interval between the atrial event and thesensed ventricular event (nodal rhythms may occur very proximate toatrial events), or the analyzing morphology and/or form of the sensedventricular event or any combinations of these methods are alltechniques available to discriminate between properly conductedventricular beats and improper beats. U.S. Pat. No. 6,029,087 issued toWohlgemuth on Feb. 22, 2000 and assigned to Vitatron, BV is hereinincorporated by reference in its entirety. The '087 patent illustratesvarious techniques for identifying sensed events based upon morphologyor form parameters and especially in the context of a digital IMD 10having digital signal processing capabilities. For purposes of thepresent invention, improper ventricular events may be identified by anyof these known techniques.

FIG. 11A is a flowchart that illustrates a process for addressing sensedimproper ventricular events. While operating in the VPP, the IMD 10 willperiodically check (600) for intrinsic conduction, as previouslyexplained. The IMD 10 will determine if any ventricular event occurs(605) during the appropriate time frame. If no ventricular event issensed (605), the VPP will operate according to the appropriateparameters, and may at some point in the future retest for intrinsicconduction (600). It should be appreciated that the VPP may ceasesubsequent conduction checks and/or may permanently exit the VPP incertain circumstances, though this is not illustrated in the flowchart.

If the IMD 10 senses a ventricular event (605) the IMD 10 will analyze(610) that event to determine if it is properly conducted. As usedherein, a regular ventricular event means a properly conductedventricular depolarization. An improper ventricular event, as usedherein means sensed ventricular activity that was not properly conductedor is improperly timed. Such improper ventricular events include PVCs,nodal rhythms and the like. If the sensed ventricular event is deemed tobe regular (615), then the intrinsic conduction check (600) is ruledsuccessful and the VPP will operate in the atrial based pacing mode(635) or in a dual chamber mode (640) with an extended AV interval(e.g., mild VPP) and proceed as previously discussed.

If the IMD 10 determines that the sensed ventricular event was improper(615), then this information is stored in memory (620). Furthermore, theintrinsic conduction test (600) is deemed failed 625; thus, the VPP willcontinue to operate in DDDR, for example. The IMD 10 analyzes (630) theinformation stored in memory regarding conduction checks failed due toimproper ventricular events. Based upon this analysis, subsequentoperation in the VPP may be modified. For example, if the impropersensed events tend to occur whenever the AV interval is permitted toextend beyond a given quantity, the VPP may modify the maximum permittedAV interval to avoid improper ventricular events. If improperventricular events are sensed each time a conduction check occurs, thenthe VPP may reduce the number of conduction check or may cease toperform them. In addition, more patient specific parameters may bedetermined. For example, the analyzed data may indicate that conductionchecks fail for improper ventricular events, if attempted within 30minutes of an elevated heart rate above some threshold. Again, the VPPwill modify the protocol to avoid conducting the conduction check inthat time frame; or, as in all of these examples, may simply report suchinformation to the physician or patient who may then alter the VPPparameters.

FIG. 11B illustrates the VPP when operating in the atrial based pacingmode (635). For each cardiac cycle, the IMD 10 will sense forventricular activity (650) over the course of the entire cycle; that is,ventricular pacing is not provided. If no ventricular event is sensed,then the VPP will follow the appropriate parameters (655), as previouslydiscussed. For example, the IMD 10 may mode switch to DDDR for one cycleto provide ventricular pacing capabilities. If there is a ventricularevent sensed (650), then the IMD 10 will analyze (660) that ventricularevent to determine (665) whether it is improper. If the ventricularevent is determined to be a properly conducted one, then the processreturns to (635) and the IMD 10 continues to operate in the atrial basedpacing mode (635).

If the ventricular event is improper (665), the IMD 10 will then responddepending upon the nature of the improper ventricular event. Twoclassifications are illustrated and are generally representative. Inthis embodiment, the IMD 10 determines (670) whether the improperventricular event is either a PVC or a nodal rhythm. Of course, thisdetermination may be made as part of the analysis (660) or at a separateprocessing point.

If the ventricular event is a nodal rhythm (670), then the IMD 10 willmode switch to a dual chamber mode such as DDDR (675). An appropriatepacing therapy is delivered (680). For example, increasing the atrialpacing rate and controlling atrial timing terminate the nodal rhythmthat is being generated within, e.g., the AV node. As proper timing isrestored, normal AV conduction and timing should return. Aftercompletion of the pacing therapy, the IMD 10 attempts to return to theatrial based pacing mode (685). If the nodal rhythm does not return,then the IMD 10 continues to operate in the atrial based pacing mode(635). If the nodal rhythm does return, then the IMD 10 provides (695)the appropriate pacing therapy to terminate the nodal rhythm. In thisinstance, the pacing regime is modified based upon the previousunsuccessful attempt. For example, the duration of the pacing therapymay be increased.

The IMD 10 will analyze (700) the nodal rhythms and the attempts attherapy and either make or suggest (e.g., provide information to aphysician) changes to the VPP as necessary. For example, nodal rhythmsmay always result in the absence of ventricular pacing for a givenpatient or if the atrial rate falls below a certain level; as such, theVPP may be set to the mild level or disabled and this could be made ratedependent. Alternatively, as indicated above, the patient may be moreprone to such nodal rhythms at specific times, such as during orfollowing periods of elevated heart rate. Thus, the VPP may operate inthe mild setting during these times. The number of attempts to terminatethe nodal rhythm required to make or suggest a given modification can beprogrammed accordingly.

If the IMD 10 determines (670) that the improper ventricular event is aPVC, that information is stored in memory (710). The IMD 10 thendetermines if this is the first PVC in a given window (715). The windowis the relevant time frame for consideration of PVCs and may be setaccordingly. In general, sporadic PVCs are non-problematic. The windowdefines the period during which multiple PVCs are sufficiently proximateone another that they may be problematic and at least warrant furtheranalysis. Thus, the window will typically be on the order of minutes,hours or multiple hours. In the present embodiment, the window includesthe previous four hours.

If the sensed PVC is the first in the window, then the IMD 10effectively ignores (720 a) the classification of improper and treatsthe ventricular event (in this instance) as a regular ventricular eventfor purposes of the VPP. Even though a PVC is not as hemodynamicallybeneficial as a properly conducted event or a paced event, theventricles do contract and provide cardiac output. While this first PVCis treated as a regular ventricular event in determining subsequentaction, the IMD 10 has identified this as a PVC which will affect theprocess if additional PVCs are sensed during the window.

In an alternative embodiment, the IMD 10 will respond to the first PVCin the window by changing (720 b) to the least aggressive VPP. In thenext cycle operated in this least aggressive VPP, the IMD 10 performs aconduction check according to the above described process. Thus, even ifthe PVC was a sporadic event, action is taken and in the next cycleventricular pacing is available. Since the conduction check is performedimmediately thereafter, the infrequent or isolated PVC will not lead tosignificant ventricular pacing. Alternatively, this conduction check mayoccur after some predetermined amount of time or number of cycles,rather than in the immediately subsequent cycle.

In another alternative embodiment, a first sensed PVC is ignored (720 c)entirely. That is, the VPP will respond to this PVC in the same mannerit would respond if no ventricular activity were sensed. Of course, theventricular event is sensed, classified as a PVC, and data indicatingthe same is retained.

Thus, the physician programmer can choose how the VPP will respond tothe first sensed PVC in a window by either treating it as a conductedevent, treating it as if no ventricular event occurred, or by taking anintermediate response with a rapid return to the standard VPP attempted.Alternatively, the IMD 10 may select which response to provide based onattempting the various responses and determining for a given patient themost successful outcome.

If the PVC is not the first in the current window (715), then the IMD 10proceeds to determine if PVCs are occurring in consecutive cardiaccycles (725). If the current PVC was preceded by a PVC in the previouscycle, then the IMD 10 determines (740) if the AV delay is greater than500 ms. If the AV delay exceeds this threshold, the presumption is thatPVCs are occurring because this delay is excessive. In response, the VPPis changed to the least aggressive (750) setting or the AV delay can beshortened. If PVCs are occurring within 500 ms or less from the atrialevent, then the cause of the PVC is not necessarily an AV interval thatis too long. As such, the VPP is changed (745) to the least aggressivemode and at the appropriate time a conduction check is performed. Ifsuccessful, the VPP can then revert to the previous level ofaggressiveness. Optionally, the atrial rate may be increased in order toavoid subsequent PVCs in some cases.

If the current PVC is not the first in the window, but does notconsecutively follow a cardiac cycle with another PVC, then the IMD 10determines (730) how many PVC have occurred over a given time period. Inthis example, the IMD 10 determines whether there have been fewer thanfive PVCs in the previous hour. If not, then the IMD 10 labels the PVCas intermittent (735) and maintains the VPP in its current form. Thoughnot illustrated, the IMD 10 may respond to this PVC in the same manneravailable for a first PVC detected in a window and undertake any ofsteps 720 a, 720 b, or 720 c.

Conversely, if the current PVC causes the PVC count to exceed thethreshold, e.g., five or more, then the IMD 10 determines (760) if aless aggressive VPP is available. If so, the VPP is changed to a VPPthat is less aggressive. If the VPP is currently in the least aggressivesetting (760), then the IMD 10 determines whether this PVC countthreshold has been exceeded (770) for some period of time. In thisembodiment, the IMD 10 determines if there have been five or more PVCsper hour for the last four consecutive hours. If this level has not beenreached, the IMD 10 continues to operate (780) in the least aggressiveVPP. Conversely, if PVC have been occurring at a rate of five or moreper hour for the last four consecutive hours, then the VPP is disabled(775) and the IMD 10 operates in a standard mode such as DDDR.

FIG. 11C illustrates a process for responding to improper ventricularevents when the initial VPP is operating with a level of aggressivenessthat provides for an extended AV interval, but not atrial based pacing.The process is substantially similar to that of FIG. 11B and only thedifferences will be discussed.

At (800 b) and (815), the AV interval is reduced since the VPP isalready in a setting where atrial based pacing is precluded. Similarly,at (820) the maximum permitted AV interval is reduced when the PVCoccurs after an AV delay of 500 ms or greater.

It should be appreciated that the VPPs as described herein performcertain functions in response to the nature of sensed cardiac activity.To facilitate explanation, these functions are sometimes described asswitching from, e.g., an atrial based mode to a dual chamber mode.Various embodiments will function in exactly that manner; that is, theoperating mode of the implantable device will switch from one cycle tothe next under the control and direction of the VPP. In otherembodiments, the same effect is achieved without switching modes in thetraditional sense. That is, an entirely new mode is provided thatprovides the same behavior or effects as described within the rules forthis new modality. Thus, device operation will change from one cycle tothe next, but the mode does not technically change. For example, such amodality is described in U.S. patent application Ser. No. 10/814,692,filed on Mar. 31, 2004 and titled Fully Inhibited Dual Chamber PacingMode, which is herein incorporated by reference in its entirety.

This application is intended to cover any adaptation or variation of thepresent invention. It is intended that this invention be limited only bythe claims and equivalents thereof.

1. A method of operating an implantable medical device (IMD) thatprovides cardiac pacing and sensing, the method comprising: operatingthe IMD in an atrial based pacing mode with a ventricular pacingprotocol (VPP), wherein the VPP is a pacing protocol that providesmultiple-beat AV coordination and minimizes ventricular pacing bytolerating a complete cardiac cycle devoid of ventricular activity whilemaintaining AV synchrony; sensing for intrinsic ventricular activity;determining the nature of the sensed ventricular activity; operatingnormally in the atrial based pacing mode if the sensed ventricularactivity is determined to be a properly conducted ventricular beat;initiating a first response under the VPP if the sensed ventricularactivity is determined to be a nodal rhythm; and initiating a secondresponse under the VPP if the sensed ventricular activity is determinedto be a premature ventricular contraction (PVC), wherein the secondresponse includes determining if the PVC is a first occurrence within apredetermined window; and operating under the VPP as if no ventricularactivity were sensed if the PVC is the first occurrence.
 2. The methodof claim 1, wherein the VPP operates in an atrial based pacing mode andchanges to a dual chamber pacing mode for one cardiac cycle immediatelyfollowing a given cardiac cycle devoid of sensed intrinsic ventricularactivity, with a return to the atrial based pacing mode immediatelysubsequent to the one cardiac cycle.
 3. The method of claim 2, whereinthe VPP further includes an aggressiveness level indicating a maximumnumber of cardiac cycles devoid of sensed intrinsic ventricular activityin a given interval tolerated by the VPP.
 4. The method of claim 1,further comprising: conducting a conduction check prior to operating inthe atrial based pacing mode to determine if intrinsic conduction ispresent; sensing for intrinsic ventricular activity; analyzing anyventricular activity sensed; failing the conduction check and operatingin a dual chamber pacing mode if no ventricular activity is sensed;operating in an atrial based pacing mode on an ongoing basis if thesensed ventricular activity is a properly conducted ventricular beat;and failing the conduction check and operating in the dual chamberpacing mode if the sensed ventricular activity is an improper event. 5.The method of claim 4, wherein the improper event is a PVC.
 6. Themethod of claim 4, wherein the improper event is a nodal rhythm.
 7. Themethod of claim 1, wherein the first response includes: operating in adual chamber mode; initiating a pacing therapy to terminate the nodalrhythm for a first duration; performing a conduction check subsequent tothe pacing therapy; and operating in the atrial based pacing mode if theconduction check is successful.
 8. The method of claim 7, furthercomprising: initiating the pacing therapy for a second duration, whereinthe second duration is longer than the first duration, if the conductioncheck fails; performing a second conduction check subsequent to thepacing therapy of the second duration; and operating in the atrial basedpacing mode if the second conduction check is successful.
 9. The methodof claim 8, further comprising discontinuing the VPP if the secondconduction check fails.
 10. The method of claim 1, wherein the secondresponse includes: determining if the PVC is a first occurrence within apredetermined window; and considering the first occurrence of a PVCwithin the window as a properly conducted ventricular event.
 11. Themethod of claim 1, wherein the second response includes: determining ifthe PVC is a first occurrence within a predetermined window; andmodifying an aggressiveness level of the VPP if the PVC is the firstoccurrence within the window.
 12. The method of claim 1, wherein thesecond response further includes: determining if the PVC was preceded byan earlier PVC in an immediately prior cardiac cycle; and changing theVPP to a least aggressive setting if the PVC was preceded by an earlierPVC in the immediately prior cardiac cycle.
 13. The method of claim 12,further comprising: determining an AV interval as defined by an atrialevent and terminated by the PVC; and returning to a more aggressive VPPsetting after one cardiac cycle in the least aggressive VPP setting ifthe AV interval is less than a predetermined duration.
 14. The method ofclaim 1, wherein the second response further includes: determining a PVCoccurrence rate for a predetermined time period; and operating the IMDin a less aggressive VPP setting if the total PVC occurrence rateexceeds a threshold.
 15. The method of claim 14, further comprising:determining if the PVC occurrence rate has exceeded the threshold formultiple consecutive predetermined time periods; and disabling the VPPif the PVC occurrence rate has exceeded the threshold for multipleconsecutive predetermined time periods.
 16. The method of claim 14,wherein disabling the VPP will only occur if the VPP is currently in aleast aggressive setting.
 17. An implantable medical device (IMD)comprising: means for cardiac sensing and pacing; means fordistinguishing regular ventricular events from improper ventricularevents; means for controlling the means for cardiac sensing and pacingaccording to a Ventricular Pacing Protocol (VPP) that respond in a firstmanner to the regular ventricular event and in a second manner for theimproper ventricular event, wherein the response in the second mannerincludes determining if the improper ventricular event is a firstoccurrence within a predetermined window; and operating under the VPP asif no ventricular activity were sensed if the improper ventricular eventis the first occurrence; wherein the VPP is a pacing protocol thatprovides multiple-beat AV coordination and minimizes ventricular pacingby tolerating a complete cardiac cycle devoid of ventricular activitywhile maintaining AV synchrony.
 18. The IMD of claim 17, whereinimproper ventricular events include premature ventricular contractions(PVC) or nodal rhythms.
 19. The IMD of claim 18, wherein the secondmanner further includes a PVC response and a nodal rhythm response.